1. Field of the Invention
The present invention relates to an optical recording apparatus for recording information by causing data light and reference light to interfere with each other inside a hologram material layer, and an optical head used therefor.
2. Description of the Related Art
In recent years, a recording medium based on holography has been under study as a novel recording medium. Such a recording medium aims at recording information by fixing, to a hologram material layer, interference fringes caused when the data light interferes with the reference light therein. Upon recording the information, the data light is subjected to spatial light modulation in accordance with information to be recorded. When the recording medium is irradiated with the data light along with the reference light, interference fringes corresponding to the information to be recorded are formed in the hologram material layer. Highly photopolymerizable monomers (with a high refractive index) are concentrated in a “bright” region in the interference fringes. Then, fixing light applied to the hologram material layer turns the highly photopolymerizable monomers concentrated in the “bright” region into polymers, and a refractive index profile is marked in conformity with the interference fringes in the hologram material layer. Information is thus recorded on the recording medium.
As known in the art regarding the above recording medium, an incident angle of the reference light to the hologram material layer is changed, information corresponding to plural pages can be multiply recorded in one recording area (multi-angle recording). That is, the data light undergoes spatial light modulation each time the incident angle of the reference light is changed for different kinds of information, whereby interference fringes each corresponding to a different piece of information to be recorded are separately fixed in the same recording area for each incident angle.
The reference light is applied to the hologram material layer with the same angle as that for information recording upon reproduction. Thus, the reference light involves coherent light corresponding to the interference fringes formed with the above angle. A light receiving element receives the thus-produced light, allowing reproduction of the recorded information with the above angle.
Note that JP 10-124872 A (Patent Document 1), JP 11-16374 A (Patent Document 2) and JP 2000-338846 A (Patent Document 3) describe an optical recording apparatus based on multi-angle recording.
In addition, JP 11-133843 A (Patent Document 4) discloses an optical recording apparatus where a special light source for applying fixing light is additionally provided aside from a light source for applying data light and reference light.
However, in the case of additionally providing the light source dedicated to the use for fixation as in Patent Document 4 above, another optical system for guiding the light from the light source to a recording medium should be provided in addition to the light source. This causes a problem in that the structure of the optical system is complicated, and the overall size of an apparatus is enlarged. In this case, also required is means for accurately applying the light from the special light source to an information-recorded area as a target of fixation. Besides, it is necessary to separately provide means for controlling positions of the recording medium and the fixing light during the relative movement therebetween.
To overcome the above problem, a method using reference light as the fixing light may be adopted. This method requires no additional light source for fixation, and is free from the above problem.
However, in this case, when entering the hologram material layer at an angle to the optical axis of the data light, the reference light undesirably reaches an information-unrecorded area other than the information-recorded area, which brings about a problem in that fixation is uselessly effected even on the information-unrecorded area exposed to the reference light. Further, a larger portion of the information-unrecorded area is exposed to the reference light when the reference light is allowed to enter the hologram material layer while changing its incident angle so as to completely fix the information over the information-recorded area without fail. As a result, the fixation is uselessly effected on the larger portion of the information-unrecorded area.